TLDR: "rm -rf ~/.var/app/com.spotify.Client/" your Spotify home dir and start over.
I just installed the latest Flatpak (from flathub) of the Spotify client (18.104.22.1686.g416cacf1) on CentOS 7. After doing this and trying to start Spotify back up, it proceeded to just open the Spotify window very quickly, and then close itself. Trying to start it up from the command line gave some more output to try to diagnose this issue.
# start the spotify client from the command line flatpak run com.spotify.Client /app/extra/bin/spotify: /app/lib/libcurl-gnutls.so.4: no version information available (required by /app/extra/bin/spotify) [spotifywm] attached to spotify [spotifywm] spotify window found
After doing this, the window was nowhere to be found. I thought it might be and issue with the file it could not find which was the "libcurl-gnutls" error above. I eventually found out this had nothing to do with the issue and it could be ignored. Eventually the only thing that fixed this issue was removing my whole Spotify home dir (which wipes out any downloaded files, settings, and your login info). Removing the dir is easy as just deleting directory "rm -rf ~/.var/app/com.spotify.Client/". Then run the command to start up the Flatpak again "flatpak run com.spotify.Client". It should start right up and you will have to login again.
Have you ever made a big playlist on Spotify, and then tried to add another song to the list that was already on it? Did Spotify tell you that song is already on the list? If your lucky it did. I've come to find out that the way Spotify checks playlists for dupes is so very basic it is only mildly helpful. I've got playlists with multiples of the same song
It seems Spotify is checking on 3 things to see if a song is a dupe. The song title, the album title and the artist name. If all 3 match, then Spotify sees it as a dupe. The problem is, songs that show up as singles don't match the album name. The same goes for the same song on an compilation album. Since the album does not match it is not seen as a duplicate by Spotify. So they happily put it on your playlist.
This is the bare minimum to do dupe checking. There are so many better ways to check for duplicate songs, and I started looking at the Spotify API to pull my playlist info to start doing my own dupe checking. Then I thought, I can't be the only who has seen this problem. One Google search later, and I found that someone has written a much better dupe checker that will also remove the songs it finds as well.The website is called Spotify Dedup. It uses Spotify's API to allow it access to your account to read and modify your playlists. This sounds shady, but it does not have access to your personal info. Just your music. After your finished removing your duplicate songs, you can revoke its access to your account. I've used it with no issues, and it does a fantastic job. The developer has a way to buy them a coffee, and I gladly did that. They saved me hours of development time to fix this glaring issue with Spotify.
Come on Spotify you can do better dupe checking. Heck, if you need the code because you just can't figure it out, Spotify Dedup shares it over on GitHub. Please fix this.